Ubuntu 17 apt, ping... suddenly started using ipv6, connections failing

Tried on MATE 17.10, Xubuntu 17.10, (and 18.04) This is affecting only some hostnames. Compare these pings to ubuntu and a ping to google:

ping -c1 ubuntu.com
PING ubuntu.com (91.189.94.40) 56(84) bytes of data.
64 bytes from ovinnik.canonical.com (91.189.94.40): icmp_seq=1 ttl=46 time=154 ms
....
1 packets transmitted, 1 received, 0% packet loss, time 0ms
ping -c1 google.com
PING google.com(sea15s01-in-x0e.1e100.net (2607:f8b0:400a:806::200e)) 56 data bytes
....
1 packets transmitted, 0 received, 100% packet loss, time 0ms

This occurs with DNS set to any of 9.9.9.9,8.8.8.8,8.8.4.4 or my ISP's DNS.

I see that many people disable IPv6. I'm not sure I want to do that. I am hoping for an explanation of this strange occurrence and for a configuration fix.

Also, without a configuration adjustment, I won't be able to update to any release updates.

Thanks.

EDIT After Soren's comment suggesting that if my ISP does not support IPv6 then my concern for keeping IPv6 is unneccessary, I felt ok about disabling IPv6. So....

disable IPv6 in Ubuntu 16+

And now apt and ping ... are working as expected. However I am still hoping for an explanation of why this started to happen.

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